Ratchets have always had an on and off switch. Generally you use the ratchet in higher winds. You turn it off for better sensitivity in lighter air. If you are sitting on the rail, the sheets may not pay out fast enough, so you turn it off. In higher winds the ratchet helps hold the sheet, but still pays out if you let it go.

I don't think there is a difference between one Harken ratchet and the next model, but more purchase will have more "holding power".